1. Field of the Invention
The present invention relates to integrated circuit structures and, in particular, to voltage stress testable embedded dual capacitor structures.
2. Description of the Related Art
An integrated circuit (IC) includes numerous electronic devices, such as bipolar transistors, metal-oxide-semiconductor (MOS) transistors, diodes, resistors and capacitors. These electronic devices are interconnected by an electrically conducting path, typically a metal line. When a capacitor is incorporated in an IC and is essential to the IC's performance and function, it is often referred to as an "embedded" capacitor. For example, an IC designed to drive a cathode ray tube (CRT), commonly referred to as a "CRT driver IC," can include such electronic devices as interconnected bipolar transistors, resistors and embedded capacitors. These IC electronic devices are adapted and arranged to provide the electronic signals that control (i.e. "drive") the CRT's electron gun. The embedded capacitor of such a CRT driver IC provides for positive feedback within the IC and, hence, increases the speed of the CRT driver IC. These embedded capacitors are, therefore, distinct from isolated test capacitors, which are not incorporated in an IC . The isolated test capacitors, consequently, do not affect the IC's performance and function. Such isolated test capacitors are described in U.S. Pat. No. 5,179,433 to Misawa et al., which is hereby incorporated by reference.
After an IC is fabricated, the functionality and reliability of the entire IC and its individual interconnected electronic devices are typically tested under a variety of electrical conditions. For the case of an IC that contains an embedded capacitor, it is frequently desirable to test the reliability of the embedded capacitor by subjecting it to a voltage stress test. In such a voltage stress test, a voltage (commonly referred to as a "applied voltage") is applied to the embedded capacitor to induce a predetermined electric field therein, while a current flow across the embedded capacitor resulting from that electric field is measured. The ICs exhibiting current flows that are greater than a predetermined current limit are rejected as unreliable, while those exhibiting current flows that are less than the predetermined current limit are considered acceptable for shipment to customers, assuming these ICs pass other appropriate functionality and reliability tests.
An obstacle, however, arises during a voltage stress test, when an embedded capacitor's reliability needs to be assured using an applied voltage at a level that is high enough to damage other electronic devices in the IC. For example, when a CRT driver IC includes a conventional single embedded capacitor that requires a voltage stress test using an applied voltage of 200 volts, but the CRT driver IC also includes interconnected bipolar transistors with breakdown voltages of less than 100 volts. To avoid damage to other electronic devices in the IC (e.g. to avoid breakdown of the interconnected bipolar transistors), conventional single embedded capacitors are regularly subjected to voltage stress tests using an applied voltage that is too low to completely guarantee their reliability.
There is, therefore, still a need in the field for an embedded capacitor structure that can be voltage stress tested using a sufficiently high applied voltage to completely assure its reliability, while not subjecting other electronic devices in the IC to a damaging level of voltage. Also needed is a process for voltage stress testing embedded capacitor structures that does not subject other electronic devices in the IC to a damaging level of voltage.